In recent years, the high-speed operation of a microprocessor is remarkable, which operation speed has increased twice as high as it was one year ago or so. In turn, the problem of the increased power consumption according to such high-speed operation has come to surface. Especially, in the cases where the microprocessor is utilized for hand-held equipments, the increased power consumption of the same shortens the duration of the batteries in use. The hand-held equipments attach importance to extending the batteries life, so that the power consumption of the processor is reduced by operating the microprocessor thereof in a lower frequency than the highest one available to the microprocessor. However, this does not allow the microprocessor to execute a high-level information processing.
Against the above inconvenience, a method to enhance the performance of the microprocessor is proposed wherein the power consumption thereof is reduced by decreasing the supply voltage and the frequency thereof where the task that the processor has to execute is of low load while by increasing the supply voltage and the frequency thereof where that is of high load.
One example of such conventional method as mentioned above is disclosed in page 292 to 293 of 2000, IEEE International Solid-State Circuits Conference, Digest of Technical Papers, which method is characterized in that the frequency and supply voltage of the microprocessor are controlled in accordance with the cut-offs as requested by the applications that the microprocessor executes and that an operating system (hereinafter referred to as OS) is utilized for assigning a frequency as required so as to constitute a system comprising a circuit, a power source and so forth for generating a supply voltage capable of executing such frequency as required.
The above prior art, as the OS is utilized for assigning an optimum frequency, occurs the following problems.    (1) The calculation function to generate a controlled optimum frequency is required for the OS. The microprocessor runs the OS itself, which causes control-related overhead to the microprocessor so as to increase its power consumption.    (2) The control method as mentioned above requires the cooperation of microprocessor and the OS, which do not operate unless both of them correspond to the same supply-voltage frequency control system. In fact, there are found the variety of the operating systems and the microprocessors on the market, which are utilized for the variety of equipments. It is predicted that the general application of such control method to such various operating systems and microprocessors may cause a problem relating to standardization.